Method and system for recycling carbon dioxide from biomass gasification

ABSTRACT

A method for recycling carbon dioxide from biomass gasification. The method includes: 1) employing carbon dioxide as a gasifying agent, allowing the carbon dioxide to gasify biomass to yield syngas; 2) cooling the syngas; 3) introduced cooled syngas to a cyclone separator and a gas scrubber for dust removal and purification; 4) allowing purified syngas in 3) to react with the vapor to modify a ratio of hydrogen to carbon monoxide of the syngas; 5) desulfurizing modified syngas to remove H 2 S and COS therein; 6) decarburizing desulfurized syngas to separate carbon dioxide therein; 7) introducing desulfurized and decarburized syngas to a synthesizing tower to yield oil products and exhaust gas including carbon dioxide; 8) decarburizing the exhaust gas including carbon dioxide and separating the carbon dioxide; and 9) introducing the carbon dioxide separated in 6) and 8) to 1) as the gasifying agent for gasification.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of International PatentApplication No. PCT/CN2013/079230 with an international filing date ofJul. 11, 2013, designating the United States, now pending, and furtherclaims priority benefits to Chinese Patent Application No.201210282152.6 filed Aug. 9, 2012. The contents of all of theaforementioned applications, including any intervening amendmentsthereto, are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method and system for recycling carbondioxide from biomass gasification.

2. Description of the Related Art

Conventional methods and systems for gasification of biomass consumeoxygen and natural gas, so that the CO₂ conversion is low, the energyconsumption is large, and the process flow is complex.

SUMMARY OF THE INVENTION

In view of the above-described problems, it is one objective of theinvention to provide a method and system for recycling carbon dioxidefrom biomass gasification. The system has high material conversion andno oxygen consumption. The method has simple process flow and zerocarbon dioxide emission.

To achieve the above objective, in accordance with one embodiment of theinvention, there is provided a method for recycling carbon dioxide frombiomass gasification, the method comprising:

-   -   1) employing carbon dioxide as a gasifying agent, allowing the        carbon dioxide to gasify biomass in a gasifier in the presence        of external auxiliary energy, whereby yielding syngas comprising        CO, CO₂, CH₄, H₂, H₂O, H₂S, and COS, wherein the gasifying agent        carbon dioxide is collected from following steps;    -   2) cooling the syngas using a primary heat exchanger and a        secondary heat exchanger in sequence, wherein the primary heat        exchanger employs carbon dioxide as a cooling medium whereby        preheating the carbon dioxide as the gasifying agent in 1), and        the secondary heat exchanger employs water as a cooling medium        whereby producing vapor;    -   3) introduced cooled syngas in 2) to a cyclone separator and a        gas scrubber for dust removal and purification;    -   4) allowing purified syngas in 3) to react with the vapor so        that part of carbon monoxide of the syngas is transformed into        hydrogen and carbon dioxide, whereby modifying a ratio of        hydrogen to carbon monoxide of the syngas;    -   5) desulfurizing modified syngas to remove H₂S and COS therein;    -   6) decarburizing desulfurized syngas to separate carbon dioxide        therein;    -   7) introducing desulfurized and decarburized syngas to a        synthesizing tower where the desulfurized and desulfurized        syngas is catalyzed to yield oil products and exhaust gas        comprising carbon dioxide;    -   8) decarburizing the exhaust gas comprising carbon dioxide and        separating the carbon dioxide, and discharging effluent gas free        of carbon dioxide; and    -   9) introducing the carbon dioxide separated in 6) and 8) to the        primary heat exchanger as the cooling medium in 2) whereby        preheating the carbon dioxide, and transporting the preheated        carbon dioxide to 1) as the gasifying agent for gasification.

In a class of this embodiment, a gasification temperature is between 600and 1300° C., and an outlet temperature of the syngas is between 700 and1100° C. Preferably, the gasification temperature is between 850 and1250° C., and the outlet temperature of the syngas is between 850 and1100° C.

In a class of this embodiment, the external auxiliary energy is plasmatorch, microwave energy, solar energy, laser energy, electric inductionenergy, or a mixture thereof, and the external auxiliary energy accountsfor 10-30% of the total energy of fuel fed to the gasifier in unit time.

In a class of this embodiment, the external auxiliary energy accountsfor 15-20% of the total energy of fuel fed to the gasifier in unit time.

In a class of this embodiment, in 1), a ratio of consumption of thecarbon dioxide to a syngas yield is between 0.36 and 0.51 under a unitstandard state; the biomass has a particle size of less than 50 mm, andthe gasifying agent carbon dioxide has a flow rate of between 30 and 60m/s. In 2), the gasifying agent carbon dioxide is preheated by theprimary heat exchanger to have a temperature of between 350 and 600° C.

In a class of this embodiment, in 4), the ratio of hydrogen to carbonmonoxide in the modified syngas is 2:1.

In another aspect, the invention provides a system for recycling carbondioxide from biomass gasification, the system comprising: a gasifier, awaste heat exchanger, a waste heat boiler, a cyclone separator, a gasscrubber, a shift reactor, a desulfurizing tower, a first decarburizingtower, a synthesizing tower, and a second decarburizing tower.

A syngas outlet of the gasifier is connected to a heat medium inlet ofthe waste heat exchanger; a heat medium outlet of the waste heatexchanger is connected to a heat source inlet of the waste heat boiler;a heat source outlet of the waste heat boiler is connected to a gasinlet of the cyclone separator; a gas outlet of the cyclone separator isconnected to an inlet of the gas scrubber; an outlet of the gas scrubberis connected to a gas inlet of the shift reactor via a compressor; and avapor outlet of the waste heat boiler is connected to a vapor inlet ofthe shift reactor.

A vapor outlet of the shift reactor is connected to an inlet of thedesulfurizing tower, and an outlet of the desulfurizing tower isconnected to an inlet of the first decarburizing tower which isconfigured for the decarburizing of the syngas; an outlet of the firstdecarburizing tower is connected to an inlet of the synthesizing tower;an exhaust outlet of the synthesizing tower is connected to an exhaustinlet of the second decarburizing tower which is configured for thedecarburizing of the exhaust; CO₂ outlets of the first decarburizingtower and the second decarburizing tower are both connected to a coldmedium inlet of the waste heat exchanger; and a cold medium outlet ofthe waste heat exchanger is connected to a gasifying agent entrance ofthe gasifier.

In a class of this embodiment, the CO₂ outlets of the firstdecarburizing tower and the second decarburizing tower are bothconnected to an inlet of the gas holder, and an outlet of the gas holderis connected to the cold medium inlet of the waste heat exchanger via ablower.

In a class of this embodiment, the inlet of the gas holder is alsoconnected to a CO₂ outlet of a calcinatory.

In a class of this embodiment, an air distributor is disposed in a lowerpart of the chamber of the gasifier; a wall of the gasifier above theair distributor comprises a primary gasifying agent entrance; the wallof the gasifier below the air distributor comprises an auxiliarygasifying agent entrance; an external auxiliary energy entrance isdisposed on the wall of the gasifier above the auxiliary gasifying agententrance; a cold medium outlet of the waste heat exchanger is connectedto both the primary gasifying agent entrance and the auxiliary gasifyingagent entrance.

Advantages according to embodiments of the invention is as follows:

1. The method employs carbon dioxide as a cycle medium, consumes zerooxygen, and discharges no carbon dioxide;

2. The method employs carbon dioxide as a gasifying agent, no oxygeninvolved, thereby supplementing the carbon source, saving the materialconsumption, and improving the conversion rate of the materials;

3. The invention has no special requirement on the particle size of thematerials, the materials merely need crushing, so the operation is easy;and

4. The external auxiliary energy can be supplied in different forms,which is beneficial to the comprehensive utilization of energy.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a system for recycling carbon dioxidefrom biomass gasification according to one embodiment of the invention;

FIG. 2 is a schematic diagram of a gasifier according to one embodimentof the invention; and

FIG. 3 is a sectional view taken from line A-A in FIG. 2.

In the drawings, the following reference numbers are used: 1. Gasifier;2. Waste heat exchanger; 3. Waste heat boiler; 4. Cyclone separator; 5.Gas scrubber; 6. Compressor; 7. Shift reactor; 8. Desulfurizing tower;9. First decarburizing tower; 10. Synthesizing tower; 11. Seconddecarburizing tower; 12. Gas holder; 13. Blower; 14. Fuel; 15. Externalauxiliary energy entrance; 16. Auxiliary gasifying agent entrance; 17.Feed inlet; 18. Syngas outlet; 19. Air distributor; 20. Primarygasifying agent entrance; 21. Slag discharging outlet; 22. Feedingdevice; 23. Slag cooler; 24. Vapor; 25. Cooled slag; 26. Fly ash; 27.Oil product; 28. Exhaust gas; 29. CO₂; 30. Syngas; 31. Effluent gas; 32.Calcinator; 33. Limestone.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Technical solution of the invention is illustrated with detailedembodiments hereinbelow, the embodiments, however, should not beexplained as limitation of the protection range of the invention.

FIG. 1 is a schematic diagram of a system for recycling carbon dioxidefrom biomass gasification of the invention in the absence of oxygen. Thesystem comprises a gasifier 1, a waste heat exchanger 2, a waste heatboiler 3, a cyclone separator 4, a gas scrubber 5, a compressor 6, ashift reactor 7, a desulfurizing tower 8, a first decarburizing tower 9,a synthesizing tower 10, and a second decarburizing tower 11. Thegasifier 1 comprises a syngas outlet 18 at the top, a slag dischargingoutlet 21 at the bottom, and a feed inlet 17 in the middle of theexternal wall of the gasifier. The feed inlet is connected to a feedingdevice 22. An air distributor 19 is disposed in a lower part of thechamber of the gasifier 1. The wall of the gasifier above the airdistributor 19 comprises a primary gasifying agent entrance 20. The wallof the gasifier below the air distributor 19 comprises an auxiliarygasifying agent entrance 16. An external auxiliary energy entrance 15 isdisposed on the wall of the gasifier above the auxiliary gasifying agententrance 16.

The syngas outlet 18 of the gasifier 1 is connected to a heat mediuminlet of the waste heat exchanger 2. A heat medium outlet of the wasteheat exchanger 2 is connected to a heat source inlet of the waste heatboiler 3. A heat source outlet of the waste heat boiler 3 is connectedto a gas inlet of the cyclone separator 4. A gas outlet of the cycloneseparator 4 is connected to an inlet of the gas scrubber 5. An outlet ofthe gas scrubber 5 is connected to a gas inlet of the shift reactor 7via a compressor 6. A vapor outlet of the waste heat boiler 3 isconnected to a vapor inlet of the shift reactor 7.

A vapor outlet of the shift reactor 7 is connected to an inlet of thedesulfurizing tower 8, and an outlet of the desulfurizing tower 8 isconnected to an inlet of the first decarburizing tower 9 which isconfigured for the decarburizing of the syngas. An outlet of the firstdecarburizing tower 9 is connected to an inlet of the synthesizing tower10. An exhaust outlet of the synthesizing tower 10 is connected to anexhaust inlet of the second decarburizing tower 11 which is configuredfor the decarburizing of the exhaust. CO₂ outlets of the firstdecarburizing tower 9 and the second decarburizing tower 11 are bothconnected to a cold medium inlet of the waste heat exchanger 2. A coldmedium outlet of the waste heat exchanger 2 is connected to both theprimary gasifying agent entrance 20 and the auxiliary gasifying agententrance 16.

The CO₂ outlets of the first decarburizing tower 9 and the seconddecarburizing tower 11 are both connected to an inlet of the gas holder12. An outlet of the gas holder 12 is connected to the cold medium inletof the waste heat exchanger 2 via a blower 13. The inlet of the gasholder 12 is also connected to a CO₂ outlet of a calcinator 32.

In this example, solid fuel 14 such as biomass is introduced from thefeeding device 22 to the gasifier 1 via the feed inlet 17. Therecyclable gasifying agent CO₂ is blown into the gasifier 1. There aretwo routes for the gasifying agent CO₂ to enter the gasifier. One isintroduced from the primary gasifying agent entrance 20, and then intothe gasifier via the air distributor 19; the other is introduced intothe gasifier 1 from the auxiliary gasifying agent entrance 16.Meanwhile, external heat energy is introduced to the gasifier via anexternal auxiliary energy entrance 15. The biomass is gasified in thegasifier 1 under high temperature to yield CO, CO₂, CH₄, H₂ andsemicoke. The reaction temperature in the gasifier is controlled atbetween 600 and 1600° C., so that the semicoke reacts with CO₂, thereaction equation is: C+CO₂=2CO+Q, with high reaction rate.

Take rice hull and 1 Nm³ of syngas as an example, the auxiliary energyaccounts for 15-25% of the total energy of the fed fuel, the reactiontemperature is 800° C., the circulating volume of CO₂ is 0.51 Nm³, thebiomass is 0.48 kg, and the syngas from the outlet of the gasifiercomprises 0-55% by volume of CO, 22-28% of CO₂, and 6-12% of H₂.

The high temperature syngas 30 is discharged from the syngas outlet ofthe gasifier 1. The cooled slag 25 is discharged from the slagdischarging outlet 21 and cooled by a slag cooler 23.

The reaction temperature in the gasifier is controlled at 600-1300° C.,preferably at 850-1250° C. The outlet temperature of the syngas iscontrolled at 800-1100° C. In the gasifier, the carrier gas of thefeedstock and the sweeping gas both employ the recyclable CO₂. Theexternal auxiliary energy accounts for 15-30% of the total energy of thefed fuel. The external auxiliary energy is any type of energy that canbe transformed into heat energy, including but not limited to plasmatorch, microwave energy, solar energy, laser energy, electric inductionenergy. The circulating volume of CO₂ can be regulated according to thefurnace temperature and fuel category. The flow rate of the gasifyingagent passing through the air distributor in the gasifier can beregulated according to the particle size of the fuel, preferably, theparticle size of the fuel is below 50 mm, and the flow rate is 30-60m/s. When the gasifier is operating, the calcinator 32 is also started,and limestone 33 is calcined to yield CO₂ to act as starting gas.

To achieve the optimal working conditions and the overall performance ofthe method, the reaction bed temperature is accurately controlled, andthe plasma power and the supplied CO₂ are real-time regulated. The abovekey parameters can be monitored by a monitoring unit disposed at thesyngas outlet of the gasifier, or by interlock control, to achieve fullautomatic operation thereby ensuring the stable running of the system.

The high temperature syngas 30 is then introduced to the waste heatexchanger 2 and exchanges heat with the gasifying agent CO₂ 29. Thus,the gasifying agent is preheated by the syngas thereby improving theconversion efficiency of the gasifier. After the primary cooling, thehigh temperature syngas is further introduced to the waste heat boiler 3and cooled therein, to yield vapor 24. After the two-stage cooling, thesyngas flows into the cyclone separator 4 and the gas scrubber 5 forfurther cooling and dust removal. The resulting fly ash 26 is collectedand discharged. The preheated CO₂ has a temperature of 350-600° C.

The cooled and scrubbed syngas is boosted by the compressor 6 and thenintroduced to the shift reactor 7, where a water gas reaction happensbetween the syngas and the vapor 24 which is originated from the wasteheat boiler 3, thereby achieving the modifying treatment of the syngasand ensuring the full utilization of the reaction products in the wholeprocess.

The modified syngas is introduced to the desulfurizing tower 8 and thefirst decarburizing tower 9 for desulfurization and decarbonization. Thepurified syngas from the first decarburizing tower 9 flows into thesynthesizing tower 10. CO₂ from the first decarburizing tower 9 flowsinto the gas holder 12 by the help of the residual pressure.

The purified syngas in the synthesizing tower 10 is transformed into oilproduces 27 through a catalytic synthesis reaction, together with thegeneration of exhaust gas 28.

The exhaust gas 28 is introduced to the second decarburizing tower 11and CO₂ 29 is separated. The remained effluent gas 31 free of greenhousegas is treated and discharged outside. Thus, the process of theinvention achieves the zero emission of greenhouse gas.

CO₂ from the first decarburizing tower 9 and the second decarburizingtower 11 are both introduced to the gas holder 12, blown by the blower13 and transported to the gasifier 1 via the primary gasifying agententrance 20 and the auxiliary gasifying agent entrance 16, and then thenext cycle of gasification starts.

While particular embodiments of the invention have been shown anddescribed, it will be obvious to those skilled in the art that changesand modifications may be made without departing from the invention inits broader aspects, and therefore, the aim in the appended claims is tocover all such changes and modifications as fall within the true spiritand scope of the invention.

The invention claimed is:
 1. A method of biomass gasification byutilizing carbon dioxide as a gasifying agent, the method comprising: 1)allowing the carbon dioxide to gasify biomass in a gasifier in thepresence of external energy, whereby yielding syngas comprising CO, CO₂,CH₄, H₂, H₂O, H₂S, and COS; 2) cooling the syngas from 1) in a primaryheat exchanger by using carbon dioxide as a cooling medium, whereby thecarbon dioxide is heated by the syngas to a temperature of between 350and 600° C.; 3) cooling the syngas from 2) in a secondary heat exchangerby using water as a cooling medium whereby producing water vapor; 4)feeding carbon dioxide from 2) to the gasifier; 5) introducing thesyngas from 3) to a cyclone separator and a gas scrubber for dustremoval and purification; 6) contacting the syngas from 5) with thewater vapor so that part of carbon monoxide in the syngas reacts withthe water vapor to produce hydrogen and carbon dioxide; 7) removing H₂Sand COS from the syngas from 6) to obtain a desulfurized syngas; 8)removing carbon dioxide from the desulfurized syngas to obtain adesulfurized and decarburized syngas; 9) introducing the desulfurizedand decarburized syngas to a synthesizing tower, wherein thedesulfurized and decarburized syngas is catalyzed to yield oil andexhaust gas comprising carbon dioxide; 10) separating carbon dioxidefrom the exhaust gas from 9) to obtain an effluent gas free of carbondioxide, and discharging the effluent gas; and 11) introducing thecarbon dioxide obtained in 8) and the carbon dioxide obtained in 10) tothe primary heat exchanger as the cooling medium.
 2. The method of claim1, wherein in 1), a gasification temperature is between 600 and 1300°C., and an outlet temperature of the syngas is between 700 and 1100° C.3. The method of claim 2, wherein in 1), the gasification temperature isbetween 850 and 1250° C., and the outlet temperature of the syngas isbetween 850 and 1100° C.
 4. The method of claim 1, wherein the externalenergy is plasma torch, microwave energy, solar energy, laser energy,electric induction energy, or a mixture thereof, and the external energyfed to the gasifier in a unit time accounts for 10-30% of the totalenergy of fuel fed to the gasifier in the unit time.
 5. The method ofclaim 3, wherein the external energy is plasma torch, microwave energy,solar energy, laser energy, electric induction energy, or a mixturethereof, and the external energy fed to the gasifier in a unit timeaccounts for 10-30% of the total energy of fuel fed to the gasifier inthe unit time.
 6. The method of claim 4, wherein the external energy fedto the gasifier in a unit time accounts for 15-20% of the total energyof fuel fed to the gasifier in the unit time.
 7. The method of claim 5,wherein the external energy fed to the gasifier in a unit time accountsfor 15-20% of the total energy of fuel fed to the gasifier in the unittime.
 8. The method of claim 1, wherein in 1), a ratio of the carbondioxide to the syngas is between 0.36 and 0.51.
 9. The method of claim1, wherein in 1), the biomass has a particle size of less than 50 mm,and the carbon dioxide has a flow rate of between 30 and 60 m/s.
 10. Themethod of claim 1, wherein in 6), a ratio of hydrogen to carbon monoxidein the syngas is 2:1 after part of carbon monoxide of in the syngasreacts with the water vapor.